Hot answers tagged coolant
I am not familiar with G12 coolant, as it seems to be VW/Audi specific, but mixing incompatible coolants will cause precipitation ("sludge") within your cooling system, which will cause blockage. G12 coolant comes in a variety of colours, read more here. What you can do is remove a small quantity of your coolant (use a turkey baster), place it in a clean ...
The most likely scenario is that you only have a single malfunction: your thermostat. If it's open all the time, the engine will take much longer to get up to temperature (as you've noticed), and some/most AC systems are completely paranoid when the engine isn't at it's operating temperature or isn't cooling normally. Get your thermostat replaced.
At the very least, the thermostat should be changed since it really isn't an expensive or intensive repair. Was this done while you were at the mechanic? This would certainly solve the temp gauge and engine temp. I don't believe (not 100% sure) this issue would affect the operation of the AC (it would with the heat).
Check for coolant in your oil too (it'll look milky or frothy). If you have that, have your car towed to your garage of choice and get your head gasket replaced. It's not safe to drive!
Nr. 1 cause: damaged head gasket Nr. 2 cause: damaged oil cooler (if present) Nr. 3 cause: cracks in the engine block/head
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